γ-Aminobutyric acid (GABA), the major inhibitory neurotransmitter in the central nervous system, is released from presynaptic inhibitory neurons and binds to chloride-selective ion channel receptors (GABAA and GABAC) and to G-protein coupled receptors (GABAB) to hyperpolarize the postsynaptic membrane, thereby controlling neuronal activity downwardly. Low levels of GABA are linked to many neurological disorders, including epilepsy, Parkinson's disease, Alzheimer's disease, Huntington's disease, and cocaine addiction.
Gabaergic drugs are those that improve secretion or transmission of GABA. These drugs as a family have been used to treat a wide variety of nervous system disorders including fibromyalgia, neuropathy, migraines related to epilepsy, restless leg syndrome, and post traumatic distress disorder. Gabaergic drugs include GABAA and GABAB receptor ligands, GABA reuptake inhibitors, GABA aminotransferase inhibitors, GABA analogs, or molecules containing GABA itself.
In 1998 a novel strategy was developed for the treatment of cocaine addiction by inhibiting the activity of γ-aminobutyric acid aminotransferase (GABA-AT), the pyridoxal 5′-phosphate (PLP)-dependent enzyme that degrades GABA. GABA-AT inhibition raises GABA levels, which antagonizes the rapid release of dopamine in the nucleus accumbens (NAcc), a neurochemical response to cocaine and other drugs of abuse. Since then, vigabatrin, the only FDA-approved inactivator of GABA-AT, which is currently used as an antiepilepsy drug, has been successful in the treatment of addiction in animal models for cocaine, nicotine, methamphetamine, heroin, and alcohol. Vigabatrin also was effective in the treatment of cocaine addiction in humans, with up to 28% of patients achieving abstinence in a 9-week double-blind trial. The potential of vigabatrin for general therapeutic use, however, may be problematic because permanent vision loss has been reported to arise from its long-term administration in 25-40% of epilepsy patients.
Recently, (1S,3S)-3-amino-4-difluoromethylenyl-1-cyclopentanoic acid, now called CPP-115, was designed, synthesized and found to be 186 times more efficient in inactivating GABA-AT than vigabatrin. (Pan, Y.; Qiu, J.; Silverman, R. B. J. Med. Chem. 2003, 46 (25), 5292-5293.) When tested in a multiple-hit rat model of infantile spasms, CPP-115 suppressed spasms at doses>100-fold lower than those used with vigabatrin and produced longer spasm suppression. CPP-115 also had a much larger margin of safety and a considerably lower retinal toxicity liability than vigabatrin. When tested in freely moving rats after administration of 20 mg/kg cocaine, CPP-115 was >300 times more potent than vigabatrin in reducing the release of dopamine in the NAcc. (Silverman, R. B. J. Med. Chem. 2012, 55 (2), 567-575; Pan, Y.; Gerasimov, M. R.; Kvist, T.; Wellendorph, P.; Madsen, K. K.; Pera, E.; Lee, H.; Schousboe, A.; Chebib, M.; Brauner-Osborne, H.; Craft, C. M.; Brodie, J. D.; Schiffer, W. K.; Dewey, S. L.; Miller, S. R.; Silverman, R. B. J. Med. Chem. 2012, 55 (1), 357-366). Also, administration of CPP-115 at 1 mg/kg, along with cocaine, to cocaine-addicted rats, showed a similar effect in eliminating their addictive behavior as vigabatrin at 300 mg/kg with cocaine.
Originally, CPP-115 was designed to inactivate GABA-AT via a Michael addition mechanism that would lead to a covalent adduct with the enzyme. However, it was later discovered that CPP-115 inactivates the enzyme by forming a tightly-bound complex with the enzyme via strong electrostatic interactions of the two carboxylate groups in the resulting metabolite with Arg192 and Arg445 in the active site (Scheme 1). (Lee, H.; Doud, E. H.; Wu, R.; Sanishvili, R.; Juncosa, J. I.; Liu, D.; Kelleher, N. L.; Silverman, R. B. J. Am. Chem. Soc. 2015, 137 (7), 2628-2640). Metabolism is initiated by Schiff base formation of CPP-115 with the lysine-bound PLP, followed by γ-proton removal and tautomerization, resulting in a Michael acceptor. However, before Lys-329 can attack this Michael acceptor, catalytic hydrolysis of the difluoromethylenyl group occurs, leading to the PLP-bound dicarboxylate metabolite, which undergoes a conformational change and tightly binds to Arg192 and Arg445 (Scheme 1). However, molecular modeling indicates a movement of the difluoromethylenyl group upon tautomerization, which bends away from Lys-329, making it too far for nucleophilic attack (Scheme 1). Instead, the enzyme presumably catalyzes hydrolysis of the difluoromethylenyl group.
